The present invention relates to an optical disk reproducing apparatus for reproducing information recorded on an optical disk, and in particular to a tracking control for correctly tracing an object track formed on an optical disk such as a compact disk (CD) loaded on an audio CD player apparatus, a CD-ROM disk loaded on a CD-ROM driving apparatus for a computer system, and a DVD disk loaded on a digital versatile disk (DVD) driving device.
FIG. 1 schematically shows a conventional configuration of a CD-ROM reproducing apparatus.
In FIG. 1, a CD-ROM optical disk 1 used as an information recording medium is loaded on a CD-ROM reproducing apparatus and rotated by a spindle motor 6 for reproducing the information recorded digitally on tracks formed on the disk 1.
An optical pickup 2 is provided as a signal extracting means for reading the information recorded on the disk 1. The optical pickup 2 emits a laser light to the track of the disk 1 and detects change in the intensity or phase of the laser light reflected from or passed through the track of the disk 1, thereby outputting an electric output signal representing the information.
A head amplifier 3 generates a tracking error signal 4 showing a positional deviation between the track and the laser light from the output signal of the optical pickup 2.
A tracking control circuit 5 receives the tracking error signal 4 to perform tracking control wherein a gain compensation and a phase compensation of a tracking servo loop including the tracking control circuit 5 are realized, so that an open loop gain and a phase margin required for the tracking servo loop can be secured. An output of the tracking control circuit 5 is applied to a tracking actuator provided in the pickup 2 via a feedback loop for performing tracking control of the pickup 2.
The tracking actuator adjusts the position of an objective lens provided on the pickup 2 finely in the direction across the track on the disk 1 so that the laser light can trace correctly the track. A motor (not shown) is also provided to move the pickup 2 in the radial direction of the disk 1.
With the feedback loop for the tracking control thus configured, the incident point of the laser light emitted from the pickup 2 is controlled so as to be kept on the track on the disk 1.
Such a tracking control performance is largely affected by an eccentricity of the disk 1. The center point of the track of the disk 1 to be reproduced and the center point of rotation of the spindle motor 6 are not coincident with each other completely, so that the disk 1 is rotated with some deviation from the rotation center or eccentricity by the spindle motor 6.
Here, referring to FIG. 2, the eccentricity of the disk rotation will be explained.
Usually, a track is formed on the disk spirally. In FIG. 2, however, for simple representation, a target track T (a solid line), the preceding and the following tracks T-1 and T+1 (broken lines) are depicted mutually concentrically on the disk, and a geometrical center of the concentric tacks is represented with A point.
When the disk is rotated centering the A point, the eccentricity does not occur, but there actually occurs some deviation in the rotation center of the disk due to mechanical factors such as a manufacturing error of a disk, and a deviation between a rotation center of a spindle motor and that of a disk and the like.
When the actual rotation center of the disk is represented with a point B, the distance between the point A and the point B represents an amount of eccentricity.
When the distance from the point A to the target track T or the radius of the track T is represented with r, the distance between the track T and the rotation center point B of the disk is not made the constant value r. That is, the distance changes by r+d at maximum and by r−d at minimum. This change occurs during one rotation of the disk and the amount of the change becomes (r+d)−(r−d)=2d.
In view of the eccentricity of the disk rotation as mentioned above, the tracking control follows up the change of the distance between the track and the actual rotation center point B so that the laser light must always be maintained on the track.
In a case of a CD-ROM reproducing apparatus, in a standard for a disk, a deviation between the center of the track and a center of a loading hole of the disk, namely the eccentricity caused by the manufacturing error of the disk, is defined as 70 μm at most. In addition to this eccentricity due to the disk manufacturing error, eccentricity such as a mechanical mounting error between the disk center and the rotation center of the disk when the disk is loaded on the spindle motor may occur, so that a total amount of eccentricity of about 200 μm may occur in an actual CD-ROM reproducing apparatus.
Also, in the standard, a distance between adjacent tracks on a CD is defined as 1.6 μm. It has been confirmed experientially that, when the tracking control can control laser light within ±0.1 μm regarding the center of a track, no problem occurs in reproducing performance.
Accordingly, in order to suppress a tracking error within 0.1 μm even if the eccentricity of 200 μm occurs as a displacement in the width direction of the track in the above-mentioned actual reproducing apparatus, an open loop gain of 20 log (200/0.1)=66.0 [dB] is required as a feedback loop for the tracking control.
FIG. 3 shows one example of open loop characteristics (frequency to gain, frequency to phase) of a tracking control loop for a CD-ROM reproducing apparatus. In the figure, the abscissa denotes a frequency of a tracking error signal supplied to the tracking control circuit 5, the ordinate in the left side denotes an open loop gain of the tracking control loop and the ordinate in the right side denotes an open loop phase deviation characteristic of the tracking control loop.
In the frequency to gain characteristic shown by a solid line in FIG. 3, the gain is flat below about 10 Hz, the gain becomes a peak in the vicinity of 40 Hz, and the gain gradually decreases above the frequency of 40 Hz.
In the frequency to phase characteristic shown in FIG. 3 with a broken line, the phase is 0 [deg] below about 10 Hz, it is −90 [deg] in the vicinity of 40 Hz, and it once becomes less than −180 [deg] in the vicinity of 50 Hz. However, the phase becomes −180 [deg] or more in a range of 700 Hz to 6000 Hz again in order to secure the phase margin. Around 2000 Hz, for example, the phase becomes −150 [deg] and the phase margin of 30 [deg] with respect to −180 [deg] can be secured.
As the influence of the eccentricity of the disk rotation appears at a frequency range in which the disk rotates. For example, a frequency in the range of 3 to 8 Hz corresponds to a normal reproducing speed in a CD-ROM reproducing apparatus. Even if an eccentricity in this frequency range is included in the tracking error signal, when the open loop gain is 66 [dB] or more, reproduction can be performed normally, even when an eccentricity of 200 μm occurs.
In a conventional tracking control, when the rotation frequency of a disk is about 100 Hz or less, a sufficient open loop gain could have been secured. However, according to a high speed reproducing in a CD-ROM reproducing apparatus, it becomes necessary to reproduce a disk at 10000 rpm, namely a rotation frequency of 166 Hz or more.
However, it becomes difficult to secure an open loop gain in a frequency range of 166 Hz or more in the conventional tracking control while maintaining the phase characteristic or phase margin by only using the conventional tracking control circuit 5 shown in FIG. 1. When it is attempted to secure only the gain sufficiently in a high rotation frequency range, the phase characteristic in other frequency ranges varies largely, so that the phase margin for maintaining a stability of the tracking servo loop can not be secured.
Consequently, with the conventional method, due to a high speed of a disk rotation, it becomes difficult to secure a stable and sufficient open loop gain as the tracking control loop, and the control performance for a disk which is made largely eccentric during rotation of the disk deteriorates. Therefore, the number of rotations for reading-out must be reduced so that a desired reading-out speed can not be realized.
In the tracking control technique for the conventional optical disk reproducing apparatus as mentioned above, there is a drawback that the control performance for a disk which is made largely eccentric during a high speed disk rotation deteriorates so that the desired reading-out speed can not be obtained.